It is well-known that an aqueous solution in contact with an untreated metal substrate can result in corrosion of the untreated metal substrate. Therefore, a metal article, such as a metal closure or container for a water-based product, like a food or beverage, is rendered corrosion resistant in order to retard or eliminate interactions between the water-based product and the metal article. Generally, corrosion resistance is imparted to the metal article, or to a metal substrate in general, by passivating the metal substrate or by coating the metal substrate with a corrosion-inhibiting coating.
Investigators continually have sought improved coating compositions that reduce or eliminate corrosion of a metal article and that do not adversely affect an aqueous product packaged in the metal article. For example, investigators have sought to improve the imperviousness of the coating in order to prevent corrosion-causing ions, oxygen molecules and water molecules from contacting and interacting with a metal substrate. Imperviousness can be improved by providing a thicker, more flexible and more adhesive coating, but often, improving one particular advantageous property is achieved at the expense of another advantageous property.
In addition, practical considerations limit the thickness, adhesive properties and flexibility of a coating applied to a metal substrate. For example, thick coatings are expensive, require a longer cure time, can be esthetically unpleasing and can adversely affect the process of stamping and molding the coated metal substrate into a useful metal article. Similarly, the coating should be sufficiently flexible such that the continuity of the coating is not destroyed during stamping and molding of the metal substrate into the desired shape of the metal article.
Investigators also have sought coatings that possess chemical resistance in addition to corrosion inhibition. A useful coating for the interior of a metal closure or container must be able to withstand the solvating properties of the packaged product. If the coating does not possess sufficient chemical resistance, components of the coating can be extracted into the packaged product and adversely affect the product. Even small amounts of extracted coating components can adversely affect sensitive products by imparting an off-taste to the product.
Organic solvent-based coating compositions provide cured coatings having excellent chemical resistance. Such solvent-based compositions include ingredients that are inherently water insoluble, and thereby effectively resist the solvating properties of water-based products packaged in the metal container.
Epoxy-based coatings and polyvinyl chloride-based coatings have been used to coat the interior of metal closures and containers for foods and beverages because these coatings exhibit an acceptable combination of adhesion, flexibility, chemical resistance and corrosion inhibition. Polyvinyl chloride-based coatings and vinyl acetate/vinyl chloride copolymer-based (i.e., solution vinyl) coatings also have been the topcoat of choice for the interior of metal closures because these coatings provide excellent adhesion to plastisol sealer gaskets applied over the cured topcoat. However, epoxy-based coatings and polyvinyl chloride-based coatings have serious disadvantages that investigators still are attempting to overcome.
For example, polyvinyl chloride-based coating compositions are thermoplastic. Thermoplastic coatings used as the topcoat of the interior coating of metal closures have inherent performance disadvantages, such as potential softening during the closure manufacturing process or under food processing conditions. Therefore, coating compositions having a thermosetting character have been investigated.
In addition, coatings based on polyvinyl chloride or a related halide-containing vinyl polymers, like polyvinylidene chloride, possess the above-listed advantageous properties of chemical resistance and corrosion inhibition, and are economical. However, curing a polyvinyl chloride or related halide-containing vinyl polymer can generate toxic monomers, such as vinyl chloride, a known carcinogen. In addition, the disposal of a halide-containing vinyl polymer, such as by incineration, also can generate toxic monomers. The generated vinyl chloride thereby poses a potential danger to workers in metal can and closure manufacturing plants, in food processing and packaging plants, and at disposal sites. Disposal of polyvinyl chloride and related polymers also can produce carcinogenic dioxins and environmentally-harmful hydrochloric acid.
Government regulators are acting to eliminate the use of polyvinyl chloride-based coating compositions that contact food, and thereby eliminate the environmental and health concerns associated with halide-containing vinyl polymers. Presently however, polyvinyl chloride-based compositions are still the predominant coating used to coat the interior of food and beverage containers and closures.
To overcome the environmental concerns and performance problems associated with polyvinyl chloride-based coating compositions, epoxy-based coating compositions recently have been used to coat the interior of food and beverage containers. However, epoxy-based coatings also possess disadvantages. For example, epoxy-based coating compositions are more expensive than polyvinyl chloride-based coating compositions.
With respect to a metal closure for a food container, the interior of a metal closure conventionally was coated with three separate coating compositions, i.e., a three-coat system. First, an epoxy/phenolic primer was applied to the metallic substrate and cured, then a vinyl-based middle coat was applied over the cured primer. Finally, after curing the middle coat, a specially-formulated topcoat capable of adhering to a plastisol sealer was applied over the cured middle coat. The plastisol sealer is applied over the cured topcoat, and formed into a gasket during manufacture of a metal closure from a metal sheet having the three cured layers of coatings applied thereon.
Two-coat systems presently are used commercially, but also exhibit disadvantages. Therefore, investigators are attempting to develop an improved two-coat system for coating the interior of a metal closure. An ideal two-coat system maintains corrosion inhibition, lowers the cost of applying the coatings, has improved rheological properties and had improved cured film integrity. Cost savings are realized both in applying one fewer coating composition to the metal substrate and in time saved by applying only two coats rather than three coats to the metal substrate.
A two-coat system for the interior of metal food closure comprises a primer (i.e., a size) and a topcoat. The metal closures typically are used in conjunction with a glass or plastic container. The topcoat must have sufficient adhesion to the primer or the coating will fail. In order to achieve sufficient intercoat adhesion, the chemical makeup of the topcoat often was dictated by the chemical makeup of the primer. Investigators therefore have been seeking a more "universal" topcoat, i.e., a topcoat that can be applied to a variety of different primers and that exhibits sufficient intercoat adhesion. Such a universal topcoat would be a significant advance in the art.
The coatings used on the interior of a metal food closure also must meet other criteria in addition to performance. For example, the coatings must incorporate components acceptable to the U.S. Food and Drug Administration (FDA) because the cured coating composition contacts food products.
The cured primer and topcoat therefore require sufficient adhesion to maintain film integrity during closure fabrication. The cured primer and topcoat also require sufficient flexibility to withstand closure fabrication. Sufficient coating adhesion and flexibility are needed for the closure to withstand processing conditions the closure is subjected to during product packaging.
Other required performance features of the cured coatings include corrosion protection and adequate adhesion to the plastisol gasket applied over the cured topcoat. Also, the cured coating composition requires sufficient chemical resistance and sufficient abrasion and mar resistance.
In the manufacture of a metal closure, a metal sheet is coated with the coating compositions, and each coating is cured individually, then the metal sheet is formed into the shape of a metal closure. The closures are made in a variety of sizes ranging from 27 mm (millimeter) to 110 mm in diameter. During manufacture, a plastisol material is molded in the form of a gasket, and typically a polyvinyl chloride-based gasket. The gasket is applied over the cured coatings on the interior of the metal closure to ensure an effective seal between the metal closure and glass container, and to maintain the vacuum condition of the packaged food product.
Product packaging is performed under processing conditions wherein the plastisol gasket is softened. When the metal closure is pressed onto the glass container, the threads on the glass container form impressions in the softened plastisol gasket. The metal closure is secured in place both by the thread impressions and by the vacuum produced by subsequent cooling. This type of metal closure is used for baby food containers and for other packaged food and beverage products, such as juices and gravies. Other types of closures are designed to be secured to glass containers by lugs rather than by thread impressions in the plastisol.
Vinyl chloride-based topcoat compositions have been softened both by product processing conditions, and by conditions encountered during closure manufacture, thereby leading to closure failure. The present invention is directed, in part, to overcoming such closure failures.
Investigators therefore have sought a two-coat system for the interior of metal closures used for vacuum-packed food products. Investigators have particularly sought a vinyl halide-free topcoat for the interior of metal closures for food and beverages that retains the advantageous properties of a vinyl chloride-based topcoat, such as adhesion, flexibility, chemical resistance, corrosion inhibition and favorable economics. Investigators especially have sought a coating composition that demonstrates these advantageous properties and also reduces the environmental and toxicological concerns associated with halide-containing vinyl polymers.
Two-coat systems have been investigated and used for application to the interior of metal closures. The investigators sought and used topcoat compositions having a sufficiently flexible cured coating such that a coated metal substrate can be deformed without destroying film continuity. The conventional epoxy resins used in the topcoats provided good adhesion to plastisol gaskets, but also often provided a rigid cured film thereby making it difficult to impossible to coat the metal substrate prior to deforming, i.e., shaping, the metal substrate into a metal article, like a metal closure. Coating a metal substrate prior to shaping the metal substrate is the present standard industrial practice.
For example, the publication P. Palackdharry et al., "Interior Two-Coat System Covers Metal Food Closures", Modern Paint and Coatings, June, 1989, pp. 78, 82 and 85, discloses a topcoat composition comprising a relatively low amount of phenolic resin, polyester and elastomer relative to the present composition. Japanese Patent JP 86/038744 discloses a coating composition for metal cans comprising a polyester, phenolic and epoxy resin. The disclosed composition also contains a polyvinyl chloride.
The above-identified patent and publication disclose coating compositions comprising an epoxy resin, a polyester and a phenolic resin. The patent and publication do not disclose a coating composition comprising an epoxy novolac resin; a polyester; a phenolic resin; and an elastomer in the amounts and ratios disclosed herein, wherein the coating composition is free of a halide-containing vinyl polymer.
Although the above-identified patent and publication disclose coating compositions for the interior of a metal food closure, the patent and publication do not disclose a topcoat composition that includes an epoxy novolac resin, a phenolic resin, a polyester and an elastomer, wherein the composition is free of a halide-containing vinyl polymer, and which, after curing, demonstrates: (1) excellent flexibility; (2) excellent adhesion to the primer coat; (3) excellent chemical resistance and corrosion inhibition; (4) excellent adhesion to the plastisol gasket; and (5) reduced environmental and toxicological concerns.
As an added advantage, a present topcoat coating composition is an improved two-coat system, thereby eliminating the time and expense attributed to applying a conventional third coat to the metal substrate. The present topcoat coating composition also can be used with a variety of types of primers without a significant decrease in coating properties.